1. Field of the Invention
The present invention relates to an image measurement for measuring the two-dimensional position and/or posture of a workpiece from an image taken by a camera.
2. Description of the Related Art
In general, it has been known that the two-dimensional positional posture of an object to be imaged (hereinafter, simply called workpiece) can be measured from an image taken by a camera. The positional posture of an actual workpiece is represented by an actual spatial coordinates (world coordinate system), while the taken image is represented by image coordinates (image coordinate system). Thus, to measure the two-dimensional positional posture of the workpiece from the taken image, information on image coordinates [pixel] is required to be associated with actual spatial coordinates [mm]. That is, an imaging plane of the camera and a measurement plane for two-dimensional measurement are identified with reference to each other.
Identification of these two planes requires calibration of internal parameters (parameters, such as the image central position, the focal length and the distortion of a lens) and the external parameters (rotational parameters and translational parameters) of the camera.
Conventionally, the external parameters are calibrated, by imaging a calibration pattern whose relative positional relationship in an actual space has been preliminarily measured, and comparing relative positional relationship between the calibration pattern on the taken image and the calibration pattern in the actual space (Japanese Patent Application Laid-Open No. 2003-50107).